Numerous on-hook/off-hook circuit state detectors are known in the art for indicating, in response to attainment of at least a predetermined minimum current level, in a telephone subscriber loop circuit that the subscriber has taken the station from the on-hook state to the off-hook state. In that state the station set hook switch is closed and telephone central office battery current can flow at a sufficient level to sustain operation of the subscriber station set for communication purposes. Some detectors of that type sense current in each line circuit conductor, i.e., the tip and ring conductors, and determine from the sensed current levels at least whether or not the subscriber station is in the off-hook state. Some examples are noted below.
A U.S. Pat. No. 4,115,664 to C. Laface et al., is directed to a loop current detecting circuit in which current is sensed and information coupled via optical couplers in each conductor to decision logic circuitry. The sensed signals are logically combined to distinguish between current flow in one or both conductors.
A telephone system supervisory circuit is shown in a U.S. Pat. No. 3,855,420 to R. E. Steidl. That circuit uses current sensing transistors to detect currents, and the combined states of the transistors define predetermined supervisory states.
In U.S. Pat. No. 4,360,709 to K. Hamazato et al., telephone loop closure detection is performed by circuits which generate signals proportional to tip and ring currents, respectively. Sensing resistors are effective in the circuit between office battery and the loop at all times. Sensing signals are summed and compared to a reference voltage to produce a signal indicative of the results of the comparison.
A hook status detector in a subscriber loop interface circuit of U.S. Pat. No. 4,406,929 to W. D. Pace et al. responds to the occurrence of loop current to develop a voltage which is compared to a reference to indicate an off-hook state. Sensing currents are coupled through series resistors which are large compared to the closed-circuit resistances usually found in loop circuits. In the off-hook state of loop operation, the sensing circuits are always energized to maintain the operative state of associated battery feed circuits.